The present invention relates to a positive-action, light-sensitive mixture which comprises a light-sensitive 1,2-naphthoquinone-2-diazide-sulfonic acid ester of a polyhydric phenol containing keto groups and which is particularly suitable for preparing planographic printing plates.
Light-sensitive compounds of this general type are disclosed in German Pat. No. 938,223 and U.S. Pat. No. 3,802,885. These are di- or tri- hydroxybenzophenones which are partially or completely esterified with naphthoquinone-diazide-sulfonic acids. Recently, the art has shown a preference amongst these compounds for the completely esterified representatives which contain no free phenolic hydroxyl groups. Such compounds exhibit relatively high light-sensitivity and give long print runs, but they have the disadvantage that their solubility in the organic solvents customarily used for coating printing plates, such as partial ethers of glycols or alkyl esters of carboxylic acids such as butyl acetate, is too low for some purposes. Moreover, in combination with the preferred binders, such as novolacs, which are insoluble in water and soluble in aqueous-alkaline solvents, they give layers which in the unexposed state are not sufficiently resistant to the usual alkaline developer solutions. These layers are also attacked to a certain extent during printing in offset printing presses, when fountain solutions containing alcohol are used.
German Offenlegungsschrift No. 2,828,,037 discloses mixtures which contain naphathoquinone-diazide-sulfonic acid esters of certain bisphenol derivatives in which two benzene rings are joined by an aliphatically substituted methylene bridge. These compounds have a high solubility in organic solvents. However, they do not have optimum resistance to alkaline developer solutions and to fountain solutions containing alcohol.